Lieve Bianca, you wrote: "I mean, we should never feel guilty for these kind of feelings, but we should think over them, because we deserve happiness",
But I think, we don't deserve happiness just like that. It is not something that grows on a tree, we have to do something ourselves, it is a mindset. When do we feel happy? For instance, when we make another person happy, we feel happy too. Happiness is not static, happiness does not come naturally. It is up to us, to make something of our lives. You cannot sit and wait till you will be happy, you have to take action.
We all have periods when we think our lives are worthless, but that also is a mindset. We are as worthless as we let ourselves think. But in reallity we are not worthless, we are very valuable, you are very valuable. The actions of one person can change the world. I don't want to go so far as to say you have to change the world (I haven't), but we can be of great value in our own surroundings, for the people we love and cherish.
I hope you will overcome these feelings my dear, In any case, I think you are very valuable!